The hidden giant: discovery of an enormous Galactic dwarf satellite in Gaia DR2

By G. Torrealba, V. Belokurov, Sergey E. Koposov, T. Li, M. Walker, J. Sanders, A. Geringer-Sameth, D. Zucker, Kyler W. Kuehn, N. Evans II and 1 more. Monthly notices of the Royal Astronomical Society, 2018

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We report the discovery of a Milky Way satellite in the constellation of Antlia. The Antlia 2 dwarf galaxy is located behind the Galactic disc at a latitude of b ∼ 11° and spans 1.26°, which corresponds to ∼2.9 kpc at its distance of 130 kpc. While similar in spatial extent to the Large Magellanic Cloud, Antlia 2 is orders of magnitude fainter at MV = −9 mag, making it by far the lowest surface brightness system known (at ∼31.9 mag arcsec−2), ∼100 times more diffuse than the so-called ultra diffuse galaxies. The satellite was identified using a combination of astrometry, photometry, and variability data from Gaia Data Release 2, and its nature confirmed with deep archival DECam imaging, which revealed a conspicuous BHB signal. We have also obtained follow-up spectroscopy using AAOmega on the AAT, identifying 159 member stars, and we used them to measure the dwarf’s systemic velocity, 290.9 ± 0.5 km s−1, its velocity dispersion, 5.7 ± 1.1 km s−1, and mean metallicity, [Fe/H] = −1.4. From these properties we conclude that Antlia 2 inhabits one of the least dense dark matter (DM) haloes probed to date. Dynamical modelling and tidal-disruption simulations suggest that a combination of a cored DM profile and strong tidal stripping may explain the observed properties of this satellite. The origin of this core may be consistent with aggressive feedback, or may even require alternatives to cold dark matter (such as ultra-light bosons).

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/ba63472cf91acd1a4794f3013515f8a916deb912

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Citations in the corpus

Milky Way Satellite Census. I. The Observational Selection Function for Milky Way Satellites in DES Y3 and Pan-STARRS DR1 (2019, methodology): “…the physical nature of such a faint system is ambiguous without internal kinematics measurements, the small physical size of this candidate is consistent with other low-luminosity outer-halo star clusters that have been discovered in recent surveys (e.g., Torrealba et al. 2019a; Mau et al. 2019).”
A Second Galaxy Missing Dark Matter in the NGC 1052 Group (2019, background): “…“ultra diffuse galaxies” (UDGs), with sizes Re > 1.5 kpc and central surface brightness µg > 24 mag arcsec−2, have been found in many different environments (including the Local Group; Martin et al. 2016; Torrealba et al. 2019), and have a wide range of properties (see, e.g., Merritt et al. 2016).”
Milky Way Satellite Census. II. Galaxy–Halo Connection Constraints Including the Impact of the Large Magellanic Cloud (2019, background): “…in the first ∼ 676 deg2 of Hyper Suprime-Cam Strategic Survey Program (HSC-SSP) imaging data (Homma et al. 2016, 2018, 2019), and by the discovery of Antlia II, the lowest surface brightness galaxy currently known, using RR Lyrae member stars identified in Gaia DR2 (Torrealba et al. 2019).”